Build some dependencies with rustpkg

Also take a rust-layers bugfix.

In collaboration with Jack Moffitt.
Pass Mac OS version cfg flags to rust-opengles build

This was in the rust-opengles Makefile.in pre-rustpkg.
Update rust-alert to find deps through rustpkg
Update mac platform submodules.
Add acid2 test case.
auto merge of #1287 : june0cho/servo/master, r=pcwalton

Upload acid2 file
add support for transparant png
Fix clean-servo to include clean-style.
Update dependencies.
Port gfx crate to new CF bindings.
Set cfg flags on all rust packages.
auto merge of #1260 : metajack/servo/kmcallister-rustpkg, r=metajack,me

Rebased from @kmcallister's branch to fix mac platform issues.
submodule updated
Remove unused files
auto merge of #1291 : sammykim/servo/unused-file, r=jdm
auto merge of #1288 : ryanhc/servo/transparent_png, r=larsbergstrom

Modified to use libpng to read .png files. We use stb_image to read all other image files.
This requires updated submodule: https://github.com/mozilla-servo/rust-png/pull/13

This patch is for:
https://github.com/mozilla/servo/issues/1279
Disable parallel builds at top level.

Sub-builds are still parallelized. This is a workaround for rustpkg
issues. See https://github.com/mozilla/rust/issues/9650
auto merge of #1292 : metajack/servo/not-parallel, r=larsbergstrom

Sub-builds are still parallelized. This is a workaround for rustpkg
issues. See https://github.com/mozilla/rust/issues/9650
Implement :root pseudo-class. Fix #1263, fix #1265
auto merge of #1293 : SimonSapin/servo/root, r=SimonSapin

Rebase/squash of @therealglazou’s #1265